One method for developing and producing hydrocarbons from deep water oil fields, is to provide a fixed platform. Such fixed platforms may have drilling equipment as well as hydrocarbon processing equipment (e.g. to separate stones, sand, etc. from hydrocarbons, separate gas from oil, and burn or reinject gas). However, in deep waters such fixed platforms are enormously expensive, with costs sometimes exceeding one billion dollars.
A lower cost approach for developing and producing from deep undersea oil fields, involves the use of a TLP (tension leg platform). A large platform floats at the sea surface and is anchored by a group of tendons that extend vertically to the seafloor. The tendons are under high tension, produced by the large buoyant platform, which results in very little drift of the platform. As wells are drilled and fluid-carrying risers are connected between the seafloor well and the platform, such risers must be placed in tension to prevent them from repeatedly scraping against one another or a tendon. Present platforms are massive, with presently installed TLP's having a net displacement of between 20,000 tons and 300,000 tons (40,000 kips to 600,000 kips, where kips stands for thousands of pounds), with the tendons producing perhaps one-quarter of the platform displacement (e.g. 5,000 to 75,000 tons). The net weight of the platform out of the water may be at least 65% to 80% of the rest of the displacement. Such large TLP's carry substantially the same type of equipment as a fixed platform, including hydrocarbon processing equipment and permanent quarters for a crew to service the various equipment. Such TLP systems may include perhaps twenty wells, and perhaps twenty corresponding fluid-carrying risers which must be tensioned.
The tension for fluid-carrying hydrocarbon-producing risers may be perhaps 100 tons each, which is much less than 1% of prior total platform displacement (at least 20,000 tons). A complement of 20 risers results in an additional downward force of perhaps 2,000 tons on the platform of the TLP system, which is no more than 10% of the platform displacement. Such relatively small riser-caused load on a prior platform may be ignored, or may be taken by an initial slightly increased tendon load. Thus, when the risers are added, perhaps one at a time or in groups of a few, there is not much effect on the system, and the system need not be compensated as risers are added. However, such systems are still expensive (even though less than a fixed platform), and a system which was of much less cost than existing TLP systems, would be of considerable value.